Small-angle scattering from molten salts

Abstract

The partial structure factors S++, S+- and 5₋in molten salts are considered at small wavenumber k and the following results are established : (i) the term of order k² in S₊₊ - S_– is determined by the difference in partial volumes of the two ionic species; (ii) a term of order k³ arises in the Sys from the presence of van der Waals dipole-dipole interactions; (iii) the charge-charge structure factor, S₊₊ + S_– - 2S_+- , is determined at order k⁴ by the true screening length of the system and affected by van der Waals interactions only at order K ⁵. Similar expansions are reported for the corresponding direct correlation functions, which are of interest for fluctuation theories of the surface properties of these liquids. All the relevant coefficients in the various expansions (both those that reflect the thermodynamic fluctuatïon behaviour of the liquid and those that reflect its microscopic behaviour) are evaluated for a charged-hard-spheres model of the family of molten alkali halides in the mean spherical approximation, using input parameters adjusted to account for the isothermal compressibility near freezing. The magnitude of the van der Waals term is also estimated.